X-Ray Absorption Fine Structure Study of Mixed Ligand Pyridine Octahedral Co(II) Complexes Having Axial Compressed/Elongated Distortion

X-ray absorption fine structure (XAFS) at the Co K-edge in three mixed ligand pyridine Co(II) complexes, viz., [Co(py)₄(SCN)₂] (1), [Co(py)₄(Cl)₂]∙H₂O (2), and [Co(py)₄(Br)₂] (3), have been investigated using synchrotron radiation. Extended X-ray absorption fine structure (EXAFS) has been analyzed using two methods to present their comparative study. In the first method, the coordination number N is fixed, and the amplitude reduction factor \({S}_{0}^{2}\) is varied. In the second method, N is varied and \({S}_{0}^{2}\) is fixed. EXAFS analysis indicated that the absorbing Co atom is surrounded by four N atoms (of pyridine ligands) in the equatorial positions forming the square planar base. Two N atoms (of thiocyanate), two chlorides, and two bromide ions in the axial sites complete the distorted octahedral coordination in complexes 1, 2, and 3, respectively. Whereas in 1, the axial bond lengths are shorter than the equatorial bond lengths, indicating compressed octahedral geometry, in 2 and 3 the axial bond lengths are longer than the equatorial bond lengths, exhibiting elongated octahedral geometry. The results agree with those obtained previously using X-ray crystallography. Using ab initio calculations, X-ray absorption near edge structure (XANES) spectra have been simulated and compared with the experimental XANES spectra. Further, p- and d-DOS calculated simultaneously to absorb the metal center of Co have been correlated with the features and shapes of the simulated XANES spectra, as well as the experimental spectra.